Introduce a flag to overwrite Date.now() with a fixed value

src/flag-definitions.h

Index: src/flag-definitions.h
diff --git a/src/flag-definitions.h b/src/flag-definitions.h
index 3ea476ae77785dd909f00bcc52ab36d47b06493a..8720ec621c0f1f8c040d27f8d76c83b4cfa7cf58 100644
--- a/src/flag-definitions.h
+++ b/src/flag-definitions.h
@@ -756,6 +756,11 @@ DEFINE_BOOL(serialization_statistics, false,
 // Regexp
 DEFINE_BOOL(regexp_optimization, true, "generate optimized regexp code")
 
+// Date
+DEFINE_INT(fixed_date, 0,
+           "Sets a fixed value for Date.now() if non-zero in seconds since the "
+           "Jan 1, 2014")
+
 // Testing flags test/cctest/test-{flags,api,serialization}.cc
 DEFINE_BOOL(testing_bool_flag, true, "testing_bool_flag")
 DEFINE_MAYBE_BOOL(testing_maybe_bool_flag, "testing_maybe_bool_flag")